The present invention relates to lithium ion secondary batteries, especially to a negative electrode for a lithium ion secondary battery and to a producing method thereof.
Lithium ion secondary batteries are gaining attention as a power source for driving electronic devices. As a negative electrode material for the lithium ion secondary battery, graphite is used mainly. However, graphite is low in capacity per unit mass, i.e., 372 mAh/g, and capacity improvement of the lithium ion secondary battery is quite difficult.
On the other hand, as a negative electrode material with higher capacity than graphite, a material that forms an intermetallic compound with lithium, such as silicon, tin, and oxides thereof, are prospective materials, for example. However, the crystal structure of these materials change while absorbing lithium to cause volume expansion. For example, when the simple substance of silicon absorbs lithium to the maximum, its volume becomes Li4.4Si. At this time, the volume expansion ratio by the charging was 4.12 times the original volume. On the other hand, the volume expansion rate in the case of graphite is 1.2 times the original volume.
The great change in volume as noted in the above causes cracks of active material particles and contact failure between the active material and the current collector, generating a possibility for shortening the charge and discharge cycle life. Particularly, when active material particles crack, the specific surface area of the active material increases, thereby promoting the reactions between the active material and non-aqueous electrolyte. As a result, the interface resistance increases due to a formation of a coating on the surface of the active material, for example, creating a cause for shortening charge and discharge cycle life.
For such problems, to secure space for easing the expansion stress and to secure current collecting performance, International Publication No. 01/031721 for example examines providing a thin film formed of an active material absorbing and desorbing lithium on the current collector, and diffusing the current collector component in the thin film, in an electrode for a lithium battery.
Japanese Laid-Open Patent Publication No. Hei 8-50922 proposes a method using an alloy formed of a metal element which forms an alloy with lithium and a metal element which does not form an alloy with lithium for the negative electrode material to minimize the cracks of the negative electrode material from the expansion and contraction stress during charge and discharge.